Dental restorative materials for use in the preparation of crowns, veneers, direct fillings, inlays, onlays and splints are known. Dental restorative materials such as dental composites contain a curable resin and particulate filler. However, resin shrinkage upon polymerization in the curing process tends to cause gap formation between the restorative composite and the tooth. As a consequence micro leakage, secondary caries and decreased longevity of the repair represents a problem with prior dental restorative materials. In order to alleviate the shrinking problem and to reinforce dental restorative materials, particulate fillers are commonly used, whereby a high filler load is preferred.
EP 1 396 254 discloses a particulate prepolymerized filler prepared by mixing an inorganic filler with an organic polymerizable resin and curing the mixture. Specifically, fumed silica and a barium aluminoborosilicate glass are mixed with polymerizable resin to prepare a paste and the paste is then heat polymerized and the resultant polymerized mass is ground to the desired particle size, for example, using a ceramic ball mill. The prepolymerized filler disclosed by EP 1 396 254 has an inorganic load of about 55 percent by volume and is used to enable higher filler loading of a dental restorative composition while maintaining acceptable handling properties of the paste.
However, the prepolymerized filler of EP 1 396 254 is problematic regarding the mechanical and chemical resistance compared to the properties of the particulate glass which is used for preparing the prepolymerized filler.
Moreover, since the prepolymerized filler must be ground and classified before incorporation into a dental restorative composition, the prepolymerized filler of EP 1 396 254 requires additional time and energy consuming process steps.
WO 0130304 discloses filler particles for use in a dental restorative composition, which comprise clusters of nano-sized metal oxide particles and further non-agglomerated nano-sized particles. The clusters are made using a process that includes heat treatment of a spray dried sol of metal oxide particles. The filler particles are silanated and incorporated into a dental restorative composition in an amount of about 78 parts by weight. WO 0130304 suggests that the clusters provide strength, while the nano-sized particles provide aesthetic quality, polishability, and wear resistance. However, the filler is problematic in that spray-drying and calcining metal oxide sol particles provide a product which requires milling in a ball mill for 160 hours in order to achieve an average cluster size of 1 μm.
The particulate filler materials of the prior art are not satisfactory when used in a dental restorative material having high filler loading for reduced shrinkage while being mechanically strong. A high filler loading results in a viscosity problem, which may usually be addressed by using large filler particles. Large filler particles generally provide a lower viscosity as compared to smaller filler particles. However, a cured product of a composition containing large filler particles is unsatisfactory regarding the polishability since a smooth surface may hardly be obtained due to large filler particles being removed from the surface of the cured product leaving cavities impairing the aesthetic properties.
U.S. Pat. No. 6,020,395 discloses a homogeneous microfilled dental composite material comprised of a mixture of polymerizable monomers and an inorganic filler, wherein said filler is comprised of silane treated fused silica aggregates having a size ranging from submicron to about 160 μm. The aggregates are comprised of agglomerates of fumed silica having an average agglomerate size in the range of approximately 0.5 to 50 μm, and the agglomerates are comprised of primary particles of fumed silica having an average particle size in the range of approximately 1 to 100 nm. The primary particles are interconnected by siloxane bridges formed by burning an organosilane coating on the fumed silica. According to Example 1, raw OX-50 was coated with 20% by weight A-174 organosilane in a V-blender using an aqueous solution spray, dried in a forced air oven at 100° C. for 24 hours, and hammermilled to a 10 μm average particle size. The silane-coated OX-50 was oxidized at 1050° C. for 4 hours, resulting in bridged silica particles (fused silica). The fused silica was then surface treated with 7% by weight A-174 organosilane in a V-blender using an aqueous solution spray. The silane-treated fused silica was dried at 110° C. for 3 hours and at 55° C. for 16 hours, then sieved through 95 mesh. The resulting filler consisted of agglomerates of Si—O bridged 0.04 μm fumed silica with aggregates of 10 μm mean size and a range of from submicron to 160 μm.
U.S. Pat. No. 4,781,940 discloses a process for the production of a filler for use in a microfilled dental composite formulation, which process comprises the steps of: (a) coating colloidal silica with a polymerizable monomer by mixing said silica with an organic solvent solution of said monomer and an effective amount of a polymerization catalyst, and then evaporating said solvent; (b) individualizing the coated silica by screening to product particles having a maximum size of about 90 microns; (c) polymerizing said monomer; and (d) individualizing the coated silica particles comprising the product of step (c) by screening. The filler of U.S. Pat. No. 4,781,940 consists of particles having an irregular shape thereby causing undesireably high viscosity increases when used in a dental composition.